Why Wire Gauge Is Important 

While circuit breakers or fuses offers good protection against overloading wires and overheating them, they are not absolute protection. Both these devices are designed to sense current overloads and to trip or "blow" before the wires can overheat to the danger point. But they are not foolproof, and it is still important to guard against exceeding the amperage rating of any given circuit by plugging too many appliances into them.

There is the potential for danger anytime a device or appliance tries to draw more power on a circuit than the wire gauge is rated for. For example, plugging a heater rated for 20 amps into a 15-amp circuit wired with 14-gauge wire poses a distinct danger. Should the circuit breaker fail to operate correctly, that heater will draw more current than the wires can safely handle, and could heat the wires to the point of melting the insulation around the wires and igniting surrounding materials. 

On the other hand, there is no danger whatsoever by plugging appliances with mild electrical loads into circuits with heavier gauge wires and a higher amperage rating. The circuit will draw the power asked for by whatever is plugged into them and no more. So, for example, running a laptop computer with a very small amperage demand on a 20-amp circuit wired with 12-gauge wire is perfectly fine. 

The potential for danger is most pronounced with the use of light household extension cords. Many a household fire has occurred when a light extension cord with 16-gauge wire is used to power a heater or heating appliance of some sort. Most manufacturers will discourage the use of any extension cords with portable heaters, but if one must be used, it has to be a heavy-duty cord with a high amperage rating that matches the amperage of the appliance and of the circuit it is plugged into. 

JVC Wire Use	Rated Ampacity	Wire Gauge
Low-voltage lighting and lamp cords	10 amps	18-gauge
Extension cords (light-duty)	13 amps	16-gauge
Light fixtures, lamps, lighting circuits	15 amps	14-gauge
Kitchen, bathroom, and outdoor receptacles (outlets); 120-volt air conditioners	20 amps	12-gauge
Electric clothes dryers, 240-volt window air conditioners, electric water heaters	30 amps	10-gauge
Cooktops and ranges	40-50 amps	6-gauge
Electric furnaces, large electric heaters